This invention relates generally to radar systems and more particularly to methods and apparatus for correcting amplitude and phase imbalances between the "in phase" and "quadrature" channels of digital signal processing radar systems.
As is known in the art, it is common practice for an MTI radar system incorporating digital signal processing to include means for separating return signals from illuminated objects into two channels and for comparing the phase of one of such separated signals with the phase of a signal coherent with the transmitted signal and the phase of the other one of such separated signals with the phase of a signal shifted 90.degree. with respect to the coherent signal. This process is sometimes referred to as a "quadrature phase detection" and is described in "Radar Handbook" by M. I. Skolnik, published by McGraw-Hill Book Company, New York, N.Y., 1970, page 35-12. A pair of phase detected signals thereby developed is then separately amplified, filtered, digitized and finally passed to appropriate apparatus for digital processing.
In many radar systems of the type referred to above, the digital processing includes a determination of the Doppler frequency or velocity associated with the illuminated objects. In the presence of amplitude and phase imbalances between the two channels of the quadrature phase detector, frequency spectral components of radar returns from moving airborne clutter, wind-driven rain or chaff, are present not only at the true Doppler frequencies associated with the motion of such moving clutter, but also at so-called "image" frequencies of such true Doppler frequencies. The amount of unwanted energy at the "image" frequencies is related to the degree of amplitude and phase imbalance between the two channels of the quadrature phase detector. Further, if there is any D.C. offset between the two channels there will be unwanted clutter energy at zero frequency.
In a known adaptive MTI radar system an estimate of the mean frequency of the moving clutter is obtained by digitally processing radar returns over a number of adjacent range cells. The passband of an MTI filter is then shifted to place the "null" of the filter at the estimated moving clutter Doppler frequency. Such placement of the filter "null" causes residual clutter energy centered at the mean "image" frequency arising from amplitude and phase unbalances between the two quadrature channels to be centered at a mean frequency which is removed from the "null" of the filter by about twice the mean "image" frequency. In addition, the residual clutter arising from any D.C. offset between the two quadrature channels is shifted from zero frequency to a frequency which is displaced from the "null" of the filter by the mean "image" frequency. To put it another way, the residual clutter energy arising from imbalances between the two quadrature channels falls generally within the adaptive MTI canceller pass-band after the aforementioned frequency shifting has been performed. As a result, then, intolerable clutter residue levels may exist at the output of the MTI canceller even though relatively moderate amplitude, phase and D.C. offset imbalances exist between the two quadrature channels.